A number of processes for preparing formaldehyde from methanol are known (see, for example, Ullmann""s Encyclopaedia of Industrial Chemistry). The processes carried out industrially are predominantly the oxidation:
CH3OH+xc2xdO2xe2x86x92CH2O+H2O
over catalysts comprising iron oxide and molybdenum oxide at from 300xc2x0 C. to 450xc2x0 C. (Formox process) and the oxidative dehydrogenation (silver catalyst process) according to:
CH3OHxe2x86x92CH2O+H2
H2+xc2xdO2xe2x86x92H2O
at from 600xc2x0 C. to 720xc2x0 C. In both processes, the formaldehyde is first obtained as an aqueous solution. Particularly when used for the preparation of formaldehyde polymers and oligomers, the resulting formaldehyde has to be subjected to costly dewatering. A further disadvantage is the formation, as by-product, of the corrosive formic acid which has an adverse effect on the polymerization.
The dehydrogenation of methanol enables these disadvantages to be avoided and, in contrast to the abovementioned processes, virtually water-free formaldehyde to be obtained directly: 
In order to achieve an ecologically and economically interesting industrial process for the dehydrogenation of methanol, the following prerequisites have to be met: The strongly endothermic reaction has to be carried out at high temperatures so as to be able to achieve high conversions. Competing secondary reactions have to be suppressed in order to achieve satisfactory selectivity to formaldehyde (without catalysis, the selectivity for forming formaldehyde is less than 10% at conversions over 90%). The residence times have to be short and the cooling of the reaction products has to be rapid in order to lessen the decomposition of the formaldehyde which is not thermodynamically stable under the reaction conditions:
CH2Oxe2x86x92CO+H2
Various methods of carrying out this reaction have been proposed; thus, for example, DE-A-37 19 055 describes a process for preparing formaldehyde from methanol by dehydrogenation in the presence of a catalyst at elevated temperature. The reaction is carried out in the presence of a catalyst comprising at least one sodium compound at a temperature of from 300xc2x0 C. to 800xc2x0 C.
J. Sauer and G. Emig (Chem. Eng. Technol. 1995, 18, 284-291) were able to set free a catalytically active species, presumed by them to be sodium, from a catalyst comprising NaAlO2 and LiAlO2 by means of a reducing gas mixture (87%N2+13%H2). This species can catalyze the dehydrogenation of methanol added downstream in the same reactor, i.e. methanol which does not come into contact with the catalyst bed, to give formaldehyde. When using nonreducing gases, only a low catalytic activity was observed.
According to J. Sauer and G. Emig and also results from more recent studies (see, for example, M. Bender et al., Presentation to the XXX. Jahrestreffen deutscher Katalytiker, Mar. 21-23, 1997), sodium atoms and NaO molecules have been identified as species emitted into the gas phase and their catalytic activity for the dehydrogenation of methanol in the gas phase has been described.
In the known processes, the starting material methanol is always reacted diluted with nitrogen and/or nitrogen/hydrogen mixtures.
Although good results are already achieved using the known processes, there is still a wide scope for improvements from a technical and economic point of view.
In various documents, for example EP-A 0 130 068, EP-A 0 261 867 and DE-A 25 25 174, it is proposed that the gas mixture formed in the reaction be used as fuel after separating off the formaldehyde.
It has now surprisingly been found that a reaction procedure which is greatly improved from a technical and economic point of view, particularly in terms of energy, can be achieved if the gas mixture formed in addition to the formaldehyde is used for diluting the starting material methanol.
The invention accordingly provides a process for preparing formaldehyde from methanol by dehydrogenation in a reactor in the presence of a catalyst at a temperature in the range from 300 to 1000xc2x0 C., wherein a circulating gas stream comprising by-products of the dehydrogenation is passed through the reactor.
The process of the invention is an ecologically and economically favorable method of producing formaldehyde having a low water content. The utilization of the hydrogen-rich by-products of the reaction, i.e. the product gas after separating off the formaldehyde, for diluting the starting material methanol for the dehydrogenation enables, on the one hand, particularly high yields to be achieved and, on the other hand, owing to the good thermal conductivity, allows the outlay in terms of apparatus for heating the starting materials, introducing the heat of reaction and cooling the products to be minimized. The further possible utilization of parts of the by-products of the reaction, i.e. the product gas after separating off the formaldehyde, as fuel for generating the reaction temperature necessary for the dehydrogenation, and also heat recovery from the waste gases, can provide the heat for this and further process steps. In this case, essentially only the desired product formaldehyde and the combustion products CO2 and H2O leave the process.
For the purposes of the invention, dehydrogenation is a non-oxidative process according to the equation: 
For the purposes of the invention, the term xe2x80x9cby-productsxe2x80x9d refers to the gas mixture which remains after separating off the product formaldehyde and comprises, apart from hydrogen, usually CO, CH4 and CO2 as well as possibly CH2O, MeOH, H2O, HCOOCH3 and/or residues from the separating off of formaldehyde, and preferably consists essentially of these gases. The ratio H2/CO in the circulating gas is particularly preferably xe2x89xa73.